This invention relates to digital communications systems and, more particularly, to a SONET data transmission system which is an optical fiber system, in which data is scrambled.
Optical fiber technology is now commonly used in data transmission systems such as telephone systems and in this system, typically, there is an optical unit which includes a fiber optic transmitter channel and a fiber optic detector channel. The fiber optic transmitter channel includes a scrambler and other modules and the fiber optic detector channel includes a descrambler, a frame detector and other modules and feeds into a multiplexer demultiplexer unit which includes an activity detector for detecting the presence of data on transmission lines and an errored frame detector.
A relatively new transmission standard has been developed for Synchronous Optical Network (SONET), which is derived from a base rate and format, combined with a multiplexing scheme. This results in a modular family of rates and formats available for use in optical interfaces. The base rate is known as the Synchronous Transport Signal level 1 (STS-1). Each STS-1 frame is a serial bit stream of 810 bytes, with each byte including eight bits. The STS-1 framing is dedicated to the first two bytes A1 and A2 of each STS-1 frame. The framing pattern for bytes A1 and A2 is F628 Hex (1111011000101000), where A1 equals F6 Hex (11110110) and A2 equals 28 Hex (00101000). A C1 identification byte follows bytes A1 and A2. Higher rates (STS-N) in SONET are accomplished by byte interleave multiplexing an N number of STS-1 frames. The framing pattern for an STS-3 frame in an Optical Carrier level 3 (OC-3) system is then A1A1A1A2A2A2. Similarly, the framing pattern for Aan STS-12 frame in an OC-12 system is twelve Al bytes followed by twelve A2 bytes.
SONET scrambling requirements are a frame synchronous scrambler of sequence length 127, operating at the line rate. SONET Specifications propose a serial scrambler where the output from the X.sup.7 position shall be added to the serial data stream. This requires the majority of the SONET interface circuitry to operate at the line rate, which in an OC-12 system is a clock rate of 622.08 MHz.
While a serial scrambler operates satisfactorily, the circuit for its implementation requires an undesirably high speed clock rate and power consumption.
It is therefore a primary object of this invention to provide a parallel scrambling circuit for use in SONET transmission systems.
Another object of this invention is to provide a scrambling circuit requiring minimal power and operating at low line rate.